Ultrasonic remote control systems are utilized in a variety of applications including remote control of television sets and remote control of underwater devices. It is common in television set remote control systems to utilize a remote transmitter which emits a different frequency for each function to be remotely controlled in the television set. The television set remote control receiving device has a number of frequency selective input circuits corresponding to the number of different frequencies. As a practical matter, the number of frequencies and, therefore, the number of functions which can be performed is limited to about a dozen whereas it may be desirable to remotely control many more functions.
A digital ultrasonic system is known for controlling underwater apparatus as described in U.S. Pat. No. 3,745,530, issued July 10, 1973, to Richard Jan Carman. In such a digital system, the transmitter transmits serially a series of pulses of one or another frequency. The underwater device may also be suitable where air is the transmission medium for the ultrasonic pressure wave. However, as recognized in that patent, a so-called "multipath" problem exists in either medium. Thus, after a data signal is transmitted, it may bounce off various objects and finally arrive at the receiver long after the data signal arrives via a direct path thus confusing the receiving equipment. The solution set forth in U.S. Pat. No. 3,745,530 is to add a filler pulse at a third frequency between adjacent data pulses to allow the multipath signal to die out before the next data pulse is transmitted. The additional filler pulses may require additional, and therefore costly circuitry to generate at the transmitter and to filter out at the receiver and may themselves create multipath problems.